Fire detection systems which respond to the sudden presence of either a fire or an explosion to thereby generate an output control signal are generally known. Such systems have a very significant utility, for example, in applications with a variety of explosive or fuel transport or storage tanks, and these systems normally function to trigger the operation of a fire suppression mechanism within a few milliseconds after the initiation of a fire or explosion. It is frequently desirable to wire these fire detectors into military armored personnel carrier vehicles which transport various arms and explosives or into rocket engines for triggering the fire suppression system and/or automatic shutdowns. A possible fire commonly desired to be suppressed by these types of fire detection systems is one which is produced in a fuel tank by a high energy round of ammunition fired into the fuel tank from a remote location. Another possible fire commonly desired to be suppressed by these types of fire detection systems is one produced by a component failure or fuel leak causing combustion to take place outside the engine.
Hitherto, fire detection and suppression systems of the above type employed one or more photon responsive short wavelength photodetectors. These photodetectors sense the energy from radiation emanating from a fire or explosion, such as red/blue or ultraviolet radiation in a particular spectral band. These systems use color (red vs. blue) comparisons, energy per time comparisons, flicker frequency comparisons or ultraviolet wavelengths alone to sense a fire. Signals from these photodetectors are properly compared and processed in order to generate a fire control output signal. A disadvantage with this type of prior art fire detection system is that the system is wholly dependent for its proper operation upon distinguishing the photon energy from the fire or explosion to be suppressed from the photon energy from the non-fire stimuli, where the non-fire stimuli can produce signals that are larger in magnitude than those of the fire stimuli. These prior art fire detection systems are frequently subject to false operation because the non-fire stimuli vary greatly from one location to another and can often deceive the fire detection systems into unwanted responses.
Various circuit techniques have been devised to discriminate against these latter sources of extraneous radiation. But these techniques have not been totally practical or satisfactory for all conditions of operation and in the many environments in which the fire detection system must be capable of operating.
In U.S. Pat. No. 3,931,521, issued Jan. 6, 1976, and assigned to the present assignee, there is disclosed a basic dual-channel fire and explosion detection system which operates to eliminate the prior art problem of false triggering in response to extraneous noise radiation in a particular spectral band. Briefly, this operation is accomplished in the above patent by the use of a long wavelength-responsive radiation detection channel and a short wavelength-responsive radiation detection channel. These two channels respond respectively to separate wavelength ranges of incident electromagnetic radiation and thereby eliminate the above possibility of false triggering, either by extraneous non-fire sources or by chopped radiation from a constant energy source, such as the sun.
U.S. Pat. No. 3,825,754, issued July 23, 1974, and assigned to the present assignee, provides further novel and useful improvements to U.S. Pat. No. 3,931,521 by providing means for discriminating between large explosive fires on the one hand and high energy flashes/explosions which cause no fire on the other. The latter could be, for example, a penetration of a High Energy Anti-Tank (HEAT) round of ammunition which does not subsequently cause a full scale explosive fire.